1. Field of the Invention
The present invention relates to a method and an apparatus for transmitting control information in wireless communication systems. More particularly, the present invention relates to a method for providing a transmission scheme that allows the transmitted signals to be received with a higher level of diversity order such that reliable transfer of information can be achieved even in mobile channels with dynamic variations in the time domain and the frequency domain.
2. Description of the Related Art
The present invention relates to a wireless cellular communication system with at least one base station (i.e., an evolved Node B (eNB)) and at least one User Equipment (UE). More particularly, the present invention relates to a wireless communication system where the eNB schedules both the downlink and uplink transmission to and from the UE. The scheduling is on a per-sub-frame basis and the scheduling indication is transmitted from the eNB to the UE via the control channel in each sub-frame of any downlink transmission.
Throughout the present invention, the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8˜10 is regarded as the legacy system and the in-development Release 11 and beyond systems are taken as a system where the exemplary embodiments of the present invention can be implemented. The present invention can also be applied to other cellular systems where appropriate.
Downlink data information is conveyed through a Physical DL Shared CHannel (PDSCH). Downlink Control Information (DCI) includes DownLink Channel Status Information (DL CSI) feedback request to UEs, Scheduling Assignments (SAs) for uplink transmission from UEs (UL SAs) or for PDSCH receptions by UEs (DL SAs). The SAs are conveyed through DCI formats transmitted in respective Physical DL Control CHannels (PDCCHs). In addition to SAs, PDCCHs may convey DCI that is common to all UEs or to a group of UEs.
In the 3GPP LTE/LTE-Advanced (LTE-A) system, the downlink transmission employs Orthogonal Frequency Division Multiple Access (OFDMA) such that the entire system bandwidth is divided into multiple subcarriers. A group of 12 consecutive subcarriers are referred to as a Resource Block (RB). An RB is the basic unit of resource allocation in the LTE/LTE-A system.
FIG. 1 is a diagram illustrating a basic unit of resource allocation in an LTE/LTE-A system according to the related art.
Referring to FIG. 1, in the time domain, the basic unit of resource allocation in the LTE/LTE-A system is the subframe. Each subframe consists of 14 consecutive OFDM symbols as shown in FIG. 1. A Resource Element is the intersection of a subcarrier and an OFDM symbol represented by a square in FIG. 1 where a single modulation symbol can be transmitted.
As shown in FIG. 1, different time and frequency resources can be used to transmit different signal types. A Cell specific Reference Signal (CRS) is transmitted to support UE mobility, such as initial access, handover operations and to support legacy PDSCH transmission modes. A Demodulation Reference Signal (DMRS) is transmitted to support new PDSCH transmission modes. Control channels are transmitted to inform the UE of the size of the control region, downlink/uplink scheduling assignments, and ACKnowlegement/Non-ACKnowlegement (ACK/NACK) for uplink Hybrid Automatic Repeat reQuest (HARQ) operations. A Channel Status Information Reference Signal (CSI-RS) is transmitted to provide UEs with a reference signal for measuring the downlink channel for CSI feedback purposes. A CSI-RS can be transmitted on any of the group of REs marked with indices A, . . . , J. Additionally, zero power CSI-RS or muting can be configured in which case the RE positions marked by indices A, . . . , J are not used for the transmission of a reference signal, a data signal, or a control signal. Zero power CSI-RS or muting is used in an LTE-A system to enhance the measurement performance of UEs receiving CSI-RS from neighboring transmission points. The PDSCH is transmitted in the data region on REs which are not used for the transmission of CRS, DMRS, CSI-RS, zero power CSI-RS.
As mentioned above, the eNB transmits PDCCH in legacy LTE/LTE-A systems for various purposes, such as an uplink/downlink scheduling assignment or a CSI feedback request indication. Due to the nature of an OFDMA system which enhances performance using frequency selective scheduling and simultaneous transmissions to multiple UEs, optimized system performance necessitates multiple PDCCHs to be transmitted to multiple UEs. Additionally, supporting Multi-User Multiple Input Multiple Output (MIMO) (MU-MIMO) where PDSCH transmissions for different UEs are spatially separated using antenna technology also requires simultaneous PDCCH transmissions to multiple UEs.
In 3GPP release 8˜10, the control channel is usually transmitted in the beginning of a sub-frame, so that the UE can efficiently acquire the scheduling information early enough for data decoding. The PDCCH is configured to be transmitted in the first one to three OFDM symbols in a sub-frame.
In order to provide the system with sufficient capacity for transmitting downlink/uplink scheduling assignments, a new Control Channel (CCH) named Enhanced Physical Data Control Channel (E-PDCCH or ePDCCH) was developed in LTE-A Release 11 to cope with the shortage of PDCCH capacity. A key factor that causes the shortage of PDCCH capacity is the fact that it is transmitted only in the first one to three OFDM symbols of a subframe. Furthermore, with frequent MU-MIMO transmissions where multiple UEs can be scheduled using the same frequency and time resources, the improvement on LTE/LTE-A systems is severely limited due to the shortage of PDCCH capacity. Unlike the PDCCH, the ePDCCH is transmitted on the data region of a subframe much like a PDSCH.
PDCCH Structure in LTE Rel8
In 3GPP LTE Release 8˜10, a PDCCH is presented in the first several OFDM symbols. The number of OFDM symbols used for PDCCH is indicated in another Physical Control Format Indication Channel (PCFICH) in the first OFDM symbol. Each PDCCH consists of L Control Channel Elements (CCEs), where L=1, 2, 4, 8 representing different CCE aggregation levels, each CCE consists of 36 sub-carriers distributed throughout the system bandwidth.
PDCCH Transmission and Blind Decoding
Multiple PDCCHs are first attached with a user-specific Cyclic Redundancy Check (CRC), independently encoded and rate matched according to CCE aggregation level 1, 2, 4 or 8, depending on link qualities, and multiplexed and mapped to the PDCCH resources. At the UE side, the UE needs to search for its PDCCHs in a pre-determined search space by assuming a certain CCE aggregation level and using the user-specific CRC. This is called blind decoding as the user may need to try multiple decoding attempts before the PDCCH could be located and identified.
Diversity Achieving Transmission Schemes
In 3GPP LTE Release 8˜10, a PDCCH is transmitted using Space Frequency Block Code (SFBC) on multiple eNB transmit antennas. SFBC is a form of transmission that allows a single modulation symbol from the UE to be received at the UE with a diversity order of two. In other words, assuming that the channel from antenna1 of the eNB to the UE is h1 and the channel from antenna2 of the eNB to the UE is h2, SFBC transmission allows the UE to recover the modulated signal which is scaled by (|h1|2+|h2|2). The received modulated signal being scaled by (|h1|2+|h2|2) means that the modulated signal has achieved a diversity order of 2. Without the use of a transmission scheme, such as SFBC, it would only be possible to achieve a diversity order of 1 in a flat fading channel. Typically, a higher diversity order would mean that the transmitted signal is more robust against wireless channel variations in the time or frequency domain. In other words, by achieving a higher diversity order, the received signal can be recovered with lower probability of error compared to the case of a lower diversity order.
SFBC in 3GPP is performed using CRS, which is a common reference signal that is used with multiple UEs connected to the same cell.
Another method of achieving diversity is by the use of delay Cyclic Delay Diversity (CDD). In 3GPP systems, large delay CDD scheme has been defined as:
      [                                                      y                              (                0                )                                      ⁡                          (              i              )                                                            ⋮                                                                y                              (                                  P                  -                  1                                )                                      ⁡                          (              i              )                                            ]    =            W      ⁡              (        i        )              ⁢          D      ⁡              (        i        )              ⁢          U      ⁡              [                                                                              x                                      (                    0                    )                                                  ⁡                                  (                  i                  )                                                                                        ⋮                                                                                            x                                      (                                          v                      -                      1                                        )                                                  ⁡                                  (                  i                  )                                                                    ]            where the precoding matrix W(i) is of size P×v, i=0, 1, . . . , Msymbap−1 is the number of antenna ports, v is the number of transmission layer, and Msymbap is the number of symbols to be precoded by the above equations. D(i) is a diagonal matrix, and U is a v×v matrix. The value of D(i) and U are predefined matrix dependent on the number of layers v.
The values of the precoding matrix W(i) are selected among the precoder elements in the codebook configured in the eNB and the UE. For 2 antenna ports, the precoder with index zero is selected. For 4 antenna ports, the UE may assume that the eNB cyclically assigns different precoders to different vectors [x(0)(i) . . . x(v−1)(i)]T on the PDSCH. A different precoder is used for every v vector. More particularly, the precoder selected according to W(i)=Ck, where k is the precoder index given by
      k    =                            (                                    ⌊                              i                v                            ⌋                        ⁢            mod            ⁢                                                  ⁢            4                    )                +        1            ∈              {                  1          ,          2          ,          3          ,          4                }              ,and C1, C2, C3, C4 denote precoder matrices corresponding to precoder indices 12, 13, 14, 15, respectively, in the four-antenna codebook. The use of large delay CDD creates an artificial delay effect on the received signal. In an OFDMA system, such delay corresponds to frequency selectivity and higher order of diversity.
DCI Transmission
A PDCCH transmission refers to a DCI transmission. There can be multiple DCIs targeting for one UE in a subframe, and a DCI could be targeting for one or multiple UEs. There are multiple types of DCI formats, among which downlink grant carries the resource allocation and transmission properties for PDSCH transmission in the present subframe, while uplink grant carries the resource allocation and transmission properties for PUSCH transmission in the uplink subframe.
PDSCH Transmission and UE-Specific Reference Signals
All those OFDM symbols after the PDCCH region can be assigned as PDSCH. The data symbols are mapped onto the sub-carriers of those OFDM symbols except the resource elements assigned for reference signals.
UE-specific reference signals, i.e., DMRS, are introduced into the system for simple implementation for beamforming transmission, where multiple antennas are precoded with different weights before transmission. In 3GPP LTE Release 8˜10, the UE-specific reference signals are precoded with the same precoder as that of the data transmitted in the same resource block. Each resource block consists of 14 OFDM symbols in the time domain and 12 subcarriers in the frequency domain. By applying the same precoding as that applied for the data transmitted on the same resource block, the UE can estimate the effect of precoding from the UE-specific reference signal without having to receive some other information which indicates the applied precoding. The UE is thus able to decode the received signals assuming the signal is transmitted from those virtual antenna ports, without knowing the exact precoder information.
FIG. 2 is a diagram illustrating DMRS ports in a resource block according to the related art.
Referring to FIG. 2, the location and port definition of DMRS in 3GPP Release 10, which can support up to eight ports from #7˜#14 is illustrated. For the case where up to 4 DMRS ports are used, ports #7/8/9/10 are spread with a spreading factor of two in the time domain. For the case where more than 4 DMRS ports are used, all ports are spread with a spreading factor of four in the time domain.
There is another subframe structure in the preferred system referred to as a Multimedia Broadcast Single Frequency Network (MBSFN) subframe, where multiple eNBs will transmit identical signaling for broadcasting purposes. A UE can be configured to receive the MBSFN subframe since not every UE is the target for MBSFN broadcasting. The system can make use of such a feature to resolve the compatibility as well as high-overhead problems when new transmission modes are introduced into the system. For example in 3GPP, the release-8 UEs will not be able to recognize the DMRS on ports 7˜14 as defined in release 10. The system can configure a subframe as a “MBSFN” subframe to release 8 UEs, while a normal subframe with only DMRS in the PDSCH region is actually transmitted for release 10 UEs who can recognize DMRS ports 7˜14 and decode data without CRS. Similar philosophy can also be applied to future evolving systems when new features are introduced.
However, in a MBSFN subframe where no CRS is defined, the legacy CDD transmission based on CRS transmission can no longer be configured. But such an open loop MIMO technique is still necessary in some scenarios when the feedback is not readily available or reliable, and/or the MIMO channel is rather selective in frequency and/or time domain.
Therefore, a need exists for a method and an apparatus for transmitting control information in wireless communication systems.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.